perf: cache CreateExecutableTestFactory delegate

[thomhurst]

Summary

• Caches the factory delegate in TestMetadata<T>.CreateExecutableTestFactory instead of creating new lambdas on every property access
• Avoids repeated delegate allocations during test discovery and execution

Fixes #4281

Test plan

• Unit tests pass (94/94)
• Benchmarks show reduced allocations during test execution

🤖 Generated with Claude Code

[thomhurst]

Summary

Caches the CreateExecutableTestFactory delegate to avoid repeated allocations during test discovery and execution.

Critical Issues

None found ✅

Suggestions

Thread Safety (Minor)

The caching pattern in TestMetadata<T>.cs:66-72 has a benign race condition - multiple threads accessing CreateExecutableTestFactory concurrently could create multiple delegate instances before caching. However, this is acceptable because:

1. The reflection mode (ReflectionTestMetadata.cs:38) uses field ??= which has the same race condition
2. The race is benign - all created delegates are functionally identical
3. Worst case is a few extra allocations during initialization, which is negligible compared to the allocations saved

If you want to eliminate the race entirely, consider using Lazy<T> or LazyInitializer.EnsureInitialized, but given the benign nature and consistency with the reflection mode, the current implementation is acceptable.

.gitignore Change

The .gitignore change adding .worktrees/ is unrelated to the performance fix but is a reasonable developer experience improvement. Consider splitting unrelated changes into separate PRs in the future for clearer history.

Verdict

✅ APPROVE - Performance improvement with dual-mode consistency. The thread safety consideration is minor and acceptable given the benign race and consistency with existing reflection mode implementation.